21332_000 2019 04 08 KJS RPT - EXO-TEC MPV Bracket Thermal Modeling.docm Page 1 TO Mr. Len Anastasi 21332.000 EXO-TEC Assembly and Bracket Modeling EMAIL len@exo-tec.biz 1 Kiddie Drive, Suite 215 Avon, MA 02322 DATE April 8, 2019 REGARDING Stand-Off MPV Bracket Three-Dimensional Thermal Modeling Report Dear Mr. Anastasi, RDH Building Science Inc. (RDH) is pleased to provide EXO-TEC Companies with this report for three-dimensional (3D) thermal modeling of your proprietary Stand-Off MPV Bracket for fastening metal panel veneer cladding systems. The purpose of this report is to present and summarize the simulated effective thermal performance results. Introduction It is widely understood that traditional methods of attaching cladding through exterior insulated wall assemblies can cause significant thermal bridging, thereby reducing the effective thermal performance of the overall wall assembly. EXO-TEC has developed an intermittent cladding support bracket known as the Stand-Off MPV Bracket to reduce thermal bridging through the insulation. See images of bracket in Figure 1. Intermittent clip systems, like the Stand-Off MPV Bracket, can reduce thermal bridging through exterior insulation when compared to more conventional solutions such as continuous vertical or horizontal Z-girts. Figure 1: Photographs of the 2” Stand-Off MPV Bracket RDH Building Science Inc. (RDH) was retained by EXO-TEC to perform three-dimensional thermal modeling of the Stand-Off MPV Bracket. Our experience with modeling of similar systems has shown 3D thermal modeling to be the most effective means of accurately simulating the thermal performance of intermittent clips. This approach provides thermal performance values suitable for use in building energy use calculations. Thermal modelling was conducted to determine clear-field effective R-values of several Stand-Off MPV bracket wall assemblies. Other performance attributes of the wall assemblies including air, water, acoustic, and fire performance were not evaluated.
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Stand-Off MPV Bracket Three-Dimensional Thermal Modeling ... · Thermal Simulation Thermal modeling was performed using HEAT3 version 9.01. HEAT3 is a three-dimensional finite element
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Thermal modeling was performed using HEAT3 version 9.01. HEAT3 is a
three-dimensional finite element thermal analysis software tool commonly used by the
building industry to analyze building assemblies in three dimensions which
two-dimensional analysis tools (such as THERM) cannot accurately analyze. It allows for
the more detailed analysis of building assemblies including the impact of thermal
bridging through walls and other penetrations.
Thermal modeling was carried out in general conformance with the ASHRAE Handbook
Fundamentals. Per industry standard modeling practices, the following assumptions have
been made:
→ Steady-state heat flow
→ Isotropic and temperature independent thermal conductivities
RDH notes that the thermal conductivity of polyisocyanurate has been shown to be
temperature dependent. The National Roofing Contractors Association (NRCA) has
performed extensive testing on the long-term thermal resistance (LTTR) of
polyisocyanurate (PIC) at different temperatures and recommends a design value of
R-5.0/in in heating dominated climates and R-5.6/in in cooling dominated climates. Other
industry references such as ASHRAE Handbook Fundamentals, present LTTR values for PIC
between R-5.6/in and R-6.5/in, with additional variance between brands. The thermal
simulations performed for this report assume a nominal PIC thermal resistance of
R-5.6/in. A design team looking to follow the NRCA recommendations regarding PIC
design thermal resistance values, can use the results for the extruded polystyrene (XPS),
which is R-5/in.
Baseline Models
A baseline model without the Stand-Off MPV Bracket was created for each type of
insulation and insulation thickness to quantify the thermal bridging impact of the bracket.
An example is shown in Figure 3.
Figure 3: 3D Thermal modelling output of a baseline model consisting of an uninsulated steel frame wall and 3-inches of exterior mineral wool insulation.
ASHRAE. (2017). Handbook Fundamentals. Atlanta: American Society of Heating, Refrigerating and Air Conditioning Engineers.
Georgia-Pacific. (2017). DensGlass Sheathing Technical Guide. Retrieved from Georgia-Pacific Building Products: http://www.buildgp.com/densglass-fiberglassmat-sheathing
NFRC. (2010). NFRC 101 Procedure for Determining Thermophysical Properties of Materials for Use in NFRC Approved Software. Greenbelt: National Fenestration Rating Council Incorporated.
NRCA. (2014). Polyiso's R-value. Industry Issue Update. Rosemont, IL: NRCA.
ROCKWOOL. (2018, January 1). CavityRock. CavityRock Technical Data Sheet. Milton: ROCKWOOL .
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Material an
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Properties
Page A.1
Standard thermal conductivity values were assumed based on NFRC 101 and ASHRAE
Handbook Fundamentals 2017. Material conductivities and their reference are shown in
Table A.1.
1 zinc & yellow chromate plated per the manufacturer’s specifications.
Table A.2 lists the boundary condition temperatures and air film thermal resistance values
for the thermal models.
TABLE A.2 BOUNDARY CONDITIONS
Material Temperature
(oF)
Resistance
(ft2 oFhr/Btu)
Interior, NFRC 100 33.8 0.68
Exterior, NFRC 100 32.0 0.17
TABLE A.1 MATERIAL CONDUCTIVITIES FOR THE CALIBRATED MODELING RESULTS
Material Conductivity
(Btu/hroFft)
Reference
Air Space varies (ASHRAE, 2017)
Aluminum Cladding 92.446 (NFRC, 2010)
Exterior Gypsum 0.074 (Georgia-Pacific, 2017)
Extruded Polystyrene 0.017 (ASHRAE, 2017)
Galvanized-Steel 35.838 (NFRC, 2010)
Interior Gypsum 0.092 (NFRC, 2010)
Mineral Wool (R-4.2/in) 0.020 (ROCKWOOL, 2018)
MPV stand-off bracket, Rolled-Steel1
28.889 (NFRC, 2010)
MPV stand-off bracket, Stainless-Steel
9.822 (NFRC, 2010)
Polycarbonate 0.116 (NFRC, 2010)
Polyisocyanurate 0.015 (NRCA, 2014)
Ap
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B
Sim
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tion
Resu
lts
Page B.1
Table B1: Effective U-values (Btu/ft2 ·°F ·hr) of rolled-steel (RS) and stainless-steel (SS) EXO-TEC MPV Stand-Off Bracket at various insulation types, depths, and bracket spacings for a steel framed wall (No Cavity Insulation).
Table B3: Chi-values (Btu/hr∙°F) of rolled steel (RS) and stainless-steel (SS) EXO-TEC MPV Stand-Off Bracket at various insulation types, depths, and bracket spacings for a steel framed wall (No Cavity Insulation).